#include<stdio.h>
int main()
{
	int grade;
	scanf("%d",&grade);
	grade /=10;
	switch(grade){
	case 10:
	case 9:
		printf("A\n");
		break;
			case 8:
		printf("B\n");
		break;
			case 7:
		printf("C\n");
		break;
			case 6:
		printf("D\n");
		break;
			default:
		printf("F\n");
		break;
	}
}